i think that happend to my kat before, and its just the milk drying up. if its hard and lumpy...... the warm washcloth or whatever will probably sooth her very good. give it a few days and see if they get smaller, and dry up... she probably over produced milk and it has no where to go, but i think it will dry up.
How do you know that the antibiotic is safe for cats? Their body chemistry is different than ours, they do NOT process meds the way we do. There aren't many safe "human" meds that cats can take.
If I were you, I would do a bunch of research-go on the search engines and type up antibiotics for cats. You should absolutely never give a cat any human med without doing research, only listen to a vet. If the VET says a human med is safe for kitty, then go for it. Otherwise do not listen to anyone that tells you otherwise-because they are clueless.
Also, keep in mind, that whatever momma kitty gets, the kittens will get through her milk.
Good luck. There are probably less ignorant vets out there that, if you call them up, they will give advice.
PLEASE, do not give the med to your cat. A lot of times it'll mess their liver and kidneys up beyond repair.